1. Field of the Invention
The present invention relates to a projection type display using a light valve, particularly to a projection type display using a transmission type liquid crystal light valve.
2. Description of the Related Art
Among projection type displays using a light valve for a light modulation, a projection type display using a liquid crystal light valve called a liquid crystal projector has the potential to replace the display device (CRT) of home televisions and personal computers (PC) in the near future because the liquid crystal projector allows the display of a fine and large image screen. Recently, due to the increase in display resolution required for PC display, finer resolution of the liquid crystal projector has been realized. The resolution has increased from the conventional 640.times.480 dots (VGA) to 800.times.600 dots (SVGA) as a standard and it will proceed to the finer 1024.times.768 dots (XGA) in future.
A schematic structure of the conventional liquid crystal projector 100 is briefly described with reference to FIG. 15. A projection optical system of the liquid crystal projector 100 comprises a lamp 2, three liquid crystal light valves 4R, 4G and 4B and a projection lens 6. Further, the projection optical system has dichroic mirrors DM1 and DM2 for separating a light from the lamp 2 into three colors of red, green and blue, dichroic mirrors DM3 and DM4 for synthesizing the separated three colors and mirrors M1 and M2. The separated three color lights are incident on the liquid crystal light valves 4R, 4G and 4B for each color respectively and are modulated according to the image signals, thereby emitting to the projection lens 6 after synthesized by the dichroic mirrors DM3 and DM4.
An image signal processing system of the liquid crystal projector 100 comprises a control unit 80 to which a image signal from the PC or the video equipment and the like inputs. The image signal inputting to the control unit is converted to a predetermined voltage and supplied to each of the liquid crystal light valves 4R, 4G and 4B. A driving voltage according to the image signal is applied to each pixel of the liquid crystal light valves 4R, 4G and 4B, thus obtaining an image on the screen by changing the transmissivity of each pixel according to the image signal and modulating the light from the lamp 2. Ordinarily, a light source capable of producing a large amount of light such as a metal halide lamp and the like is used as the lamp 2. Therefore, a large electric power is supplied from a power supply 26 and the lamp 2 is heated up to a high temperature.
The heat produced at the lamp 2 increases an internal temperature of the body of the liquid crystal projector 100 by radiation or heat conduction through air. Further, a liquid crystal in the liquid crystal panel structuring the liquid crystal light valve and a polarizing plate attached on a surface of the liquid crystal panel and the like increase the temperature for themselves by absorbing the light. In conventional liquid crystal projector 100 air flows around the liquid crystal light valves 4R, 4G and 4B so that the temperatures of the liquid crystal and the polarizing plate are maintained within a specification temperature determined, for example, at approximately 60.degree. C. An intake fan 10 and an exhaust fan 12 are provided on the body of the liquid crystal projector 100. In FIG. 15, the intake fan 10 is attached in the plane direction of the body and shown by dashed lines. The exhaust fan 12 is provided on the side of the body. An enforced cooling for making a air flow around the liquid crystal light valves 4R, 4G and 4B is performed by rotating these fans 10 and 12. Further, as shown in circular dashed lines in the figure, a filter 14 is mounted on the air inflow side of the intake fan 10 to prevent the dust from entering.
Also, if the temperature of the surrounding environment increases when the apparatus is working, or the filter 14 provided on the intake fan 10 for taking in the external air clogs due to dust and the like, the internal temperature of the apparatus extraordinarily increases, so that the members in the body, specifically, the structure members of the liquid crystal light valve may produce a deterioration in reliability. To avoid this, functions for alerting the operator and automatically stopping the apparatus by detecting the extraordinary temperature are provided to the control unit 80 of the liquid crystal projector 100.
Therefore, a temperature detecting element 30 for detecting the temperatures of the liquid panel and the polarizing plate of the liquid crystal light valve 4G is attached in the vicinity of the liquid crystal light valve 4G. The detecting signal from the temperature detecting element 30 is output to the control unit 80 through a signal line. The control unit 80 compares the temperature detecting signal from the temperature detecting element 30 with the pre-memorized reference value. When the temperature detecting signal exceeds a reference value, the lamp 2 is disconnected or the power supply 26 of the liquid crystal projector 100 is disconnected. In the past, an allowable temperature at the liquid crystal members or the polarizing plate of the liquid crystal light valves 4R, 4G and 4B is set as the reference value of an apparatus specification temperature. When this reference value is exceeded, the power supply 26 is disconnected and lamp 2 is also disconnected.
A thermistor is, for example, used as the temperature detecting element 30 for detecting the extraordinary temperature in the projector 1. Though the temperature detecting element 30 is away from the position where the liquid crystal panel and the polarizing plate become the maximum temperature, it is arranged at the downwind side of the liquid crystal panel as a position which can detect the temperature as close as possible to the maximum temperature. Since the temperature detecting element 30 is away from the maximum temperature point of the members which are monitored, the temperature at the element 30 is increased less due to cooling of from maximum temperature point. Nonetheless, the temperature due to the heat transmitted from the members is actually measured where the temperature detecting element 30 is located.
Here, an operation for disconnecting the power source 26 and the like is described using a flow chart shown in FIG. 16. In FIG. 16, first, a temperature t .degree. C. in the vicinity of the liquid crystal light valve 4G based on the output from the temperature detecting element 30 (step S100). Next, by comparing a predetermined temperature t.sub.OFF for disconnecting the power source 26 with the measured temperature t (step S101). When the temperature t is larger than t.sub.OFF (t&gt;t.sub.OFF), in short, the temperature t exceeds the temperature for disconnecting the power source 26, and the process proceeds to step S102 to process the disconnection of the power source 26. When the temperature t is not larger than t.sub.OFF (t.sub.C =&lt;t.sub.OFF), the process proceeds to step S103 because disconnection of the power source 26 is not required. At step S103, a predetermined temperature t.sub.ALARM for displaying an alarm and the measured temperature t are compared. When the temperature t is larger than the temperature t.sub.ALARM (t&gt;t.sub.ALARM), in short, the temperature t exceeds the temperature that triggers the alarm, and process proceeds to step S104 to trigger the alarm and then return to step S100. When the temperature t is not larger than t.sub.OFF a (t=&lt;t.sub.OFF) at step S103, the alarm display is turned off at step S105 and the process moves to the control of the fans 10 and 12 (step S106). Thereafter, whether or not to complete operation is determined. When the operation is continued, the step returns to step S100 and a new temperature t is acquired.
Thus, the conventional liquid crystal projector 100 measures the temperature in the vicinity of the liquid crystal panel by, for example, the temperature detecting element 30 arranged at the liquid crystal light valve 4G and disconnect the power source 26 and the lamp 2 by comparing the pre-memorized reference value with the measured results to avoid the deterioration or malfunctioning of the liquid crystal or the polarizing pane caused by the heat or light from the lamp 2.
In the meantime, the liquid crystal projector 100, can be for example, fully considered to be used at the position where an altitude is high and an air pressure is low. Since the air pressure decreases and the air is low in density at higher altitudes, when the fans 10 and 12 rotate at the same fan rotational frequency as one at lower altitudes, the cooling effect with respect to the liquid crystal light valves 4R, 4G and 4B reduces. Therefore, the temperatures of the liquid crystal light valves 4R, 4G and 4B reduce. On the contrary, since heat conductivity is reduced by the extent of the reduced density due to the low pressure, the detecting temperature by the temperature detecting element 30 such as a thermistor does not increase as much. In short, the adjustment of temperature detecting sensitivity of the temperature detecting element 30 at the reference altitude (air pressure) does not lead to an accurate measurements of the temperatures of the liquid crystal light valves 4R, 4G and 4B because the detecting temperature based on the output from the temperature detecting element 30 changes when the liquid crystal projector 100 is carried and used at the position higher (or lower) than the reference altitude.
When the liquid crystal projector 100 is used at a position higher in altitude than the reference position, the temperature detecting element 30 detects lower temperatures than the actual temperatures of the liquid crystal light valve. Therefore, the alarm display for alerting an operator or the function for disconnecting the power source 26 and the lamp 2 in case of emergency may not work normally.
On the other hand, when case the sensitivity of the temperature detecting element 30 is adjusted by assuming that the reference altitude is at the predetermined higher position, if the apparatus is used at the lower position than the reference position having the higher air pressure, the functions for the alarming display or for disconnecting the power source 26 and the lamp 2 work even if the temperature is lower than one required evoke these functions. This creates a situation where the expected apparatus specification can not be satisfied.
Further, when the air pressure is low and the environment temperature is high, or the filter 14 is clogged, the internal temperature of the liquid crystal projector 100 increases. In this case, the alarm display detects the extraordinary temperature of the projector 100 or an automatic disconnection of the power source 26 of the projector 100 occurs at the environment temperature lower than the environment temperature which is set at the reference altitude in the apparatus specifications. The environment temperature at which the power source 26 is disconnected is at least required to be equal to the sum of the upper limit value of the specification of the environment temperature and a temperature margin due to the mechanical member intrinsic error. The internal temperature of the apparatus detected, based on the output from the temperature detecting element, 30 becomes the reference for a decision.
Therefore, it is desirable to set the detecting temperature t.sub.OFF, based on the output from the temperature detecting element 30 for disconnecting the power source 26 when the internal temperature of the apparatus increases extraordinarily, as t.sub.OFF =t.sub.UP +.alpha. in point for protecting the materials from heat. Here, t.sub.UP is the upper limit of the temperature characteristics under the apparatus working environment and .alpha. is a necessary margin of the machine error or the like.
However, since the situation of t.sub.OFF &lt;t.sub.up +.alpha. occurs when the air pressure reduces as above-mentioned, the characteristic of the apparatus working environment temperature is not satisfied. Therefore, though an air pressure variation margin .beta. is also required for the design of t.sub.OFF, if the .beta. is assumed as a fixed value, the margin becomes surplus under a high air pressure environment and the apparatus may be used at higher temperature condition than intended, thereby causing a high burden on the optical members and reduction in reliability.